Pedicle screws and methods of using the same

ABSTRACT

Pedicle screws that can include a pedicle screw body, housing, plurality of clamps, rod, and set screw are disclosed herein. The clamps may be positioned side by side inside the lower portion of the housing. When the rod and set screw are provided in the housing, the set screw applies a force on the rod and the rod engages the plurality of clamps, causing the clamps to frictionally engage a head of the pedicle screw between the clamps.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/571,723, filed on Aug. 10, 2012, which is a continuation of U.S.patent application Ser. No. 12/510,897, filed on Jul. 28, 2009, whichclaims the priority benefit under 35 U.S.C. §119(e) of U.S. ProvisionalApplication Ser. No. 61/162,113, filed on Mar. 20, 2009, each of whichis incorporated herein by reference in its entirety for all purposes.

BACKGROUND

1. Field of the Invention

Disclosed herein are pedicle screws related to the field of orthopedicsurgery. More particularly, certain embodiments disclosed herein relateto a pedicle screw and methods of use.

2. Description of the Related Art

Spinal fusion encompasses a surgical technique in which two or morevertebrae are connected together. This technique may be used formultiple indications, including abnormal spinal curvature (e.g.,scoliosis) and weakening or injuring of the vertebrae or spinal disc.

In some instances, this process is accomplished and/or supplementedusing a plurality of screws implanted into the pedicle of adjacentvertebrae and joined together by a series of one or more rods. Thepedicle screw may have an enlarged head that interfaces with a housinghaving a corresponding cavity, thus allowing for a range of polyaxialarticulation between the screw and the housing. After the pedicle screwis implanted into bone, a rod may be placed in the housing, and a setscrew may delivered into engagement with the housing, applying adownward force on the rod to hold the assembly together. This downwardforce is also supposed to cause engagement of the head of the pediclescrew relative to the housing, to thereby fix the position of thepedicle screw relative to the housing and prevent disengagement.However, prior pedicle screw designs may not provide sufficient force tohold the pedicle screw relative to the housing to fix their relativelocations and/or prevent disengagement.

SUMMARY

Described herein are pedicle screws that can include a screw body, aplurality of clamps, a housing, and a set screw. Methods of assemblingand implanting the pedicle screws described herein are also included.These pedicle screws and related methods are described in greater detailbelow.

Some embodiments herein are directed to a pedicle screw that can includea threaded screw body having a proximal end and a distal end, theproximal end including an at least partially ball-shaped member; ahousing having an upper portion with an upper opening and a lowerportion with a lower opening extending along a first axis of thehousing, the upper portion being internally threaded and the lowerportion having a curved internal surface, and a third opening and afourth opening along a second axis transverse to the first axis adaptedto receive an elongated rod; a plurality of clamps positionedside-by-side adjacent the lower portion of the housing, each of theclamps having an upper portion with a cylindrical outer surface and alower portion with a curved outer surface and a ball-shaped innersurface, the curved outer surface of the lower portion adapted to engagethe curved internal surface of the lower portion of the housing, theupper portion of the clamps adapted to contact the elongated rod, andthe ball-shaped inner surface adapted to engage the ball-shaped member;and a set screw configured to be received within the internally threadedfirst portion of the housing, wherein the set screw when engaged in thefirst portion applies a force onto the rod when received in the housing,the rod engaging the plurality of clamps to frictionally engage theball-shaped member against the ball-shaped inner surface of each of theclamps.

In some embodiments, the ball-shaped inner surface includes a pluralityof grooves. In some embodiments, the diameter of the ball-shaped memberis less than the inner diameter of the lower opening of the housing. Insome embodiments, the pedicle screw includes no more than two clamps. Insome embodiments, each clamp has an upper portion with an indentationshaped to receive the rod. In some embodiments, the pedicle screwfurther includes a crimp from the outside of the housing to hold theclamps within the housing. In some embodiments, the radius of curvatureof the curved outer surface of the lower portion of each of the clampsis different from the radius of curvature of the curved internal surfaceof the lower portion of the housing.

Some embodiments herein are directed to a pedicle screw that can includea threaded screw body having a proximal end and a distal end, theproximal end including an enlarged head; a housing having an upperportion having an upper opening and a lower portion having a loweropening extending along a first axis of the housing, the upper portionbeing internally threaded and the lower portion having a graduallydecreasing dimension toward the lower opening, and a third opening and afourth opening along a second axis transverse to the first axis adaptedto receive an elongated rod; a plurality of clamps positionedside-by-side adjacent the lower portion of the housing, each of theclamps having an upper surface adapted to engage the elongated rod and alower surface adapted to engage the enlarged head; and a set screwconfigured to be received within the internally threaded first portion,wherein the set screw when engaged in the first opening applies a forceonto the rod when received in the housing, the rod engaging theplurality of clamps and causing the clamps to frictionally engage theenlarged head between the lower surfaces of the clamps.

Some embodiments herein are directed to a pedicle screw that can includea threaded screw body having a proximal end and a distal end, theproximal end including an at least partially ball-shaped member; ahousing having an upper portion with an upper opening and a lowerportion with a lower opening extending along a first axis of thehousing, the upper portion being internally threaded and the lowerportion having an internal surface, and a third opening and a fourthopening along a second axis transverse to the first axis adapted toreceive an elongated rod; a plurality of clamps positioned side-by-sideadjacent the lower portion of the housing, each of the clamps having anupper portion and a lower portion, the lower portion having an outersurface and a ball-shaped inner surface, the outer surface of the lowerportion adapted to engage the internal surface of the lower portion ofthe housing, the upper portion of the clamps adapted to contact theelongated rod, and the ball-shaped inner surface adapted to engage theball-shaped member; and a set screw configured to be received within theinternally threaded first portion of the housing, wherein the set screwwhen engaged in the first portion applies a force onto the rod whenreceived in the housing, the rod engaging the plurality of clamps tofrictionally engage the ball-shaped member against the ball-shaped innersurface of each of the clamps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of an embodiment of a pedicle screw disclosedherein.

FIG. 2 is a side view of a screw body as disclosed herein.

FIG. 3 is a cross-sectional view of an embodiment of a housing of apedicle screw disclosed herein.

FIG. 4 is a side view of an embodiment of a housing of a pedicle screwdisclosed herein.

FIG. 5 is a cross-sectional view of a pair of clamps as disclosedherein.

FIG. 6 is a cross-sectional view of the pair of clamps of FIG. 5, takenthrough line 6-6.

FIG. 7 is a cross-sectional view of an embodiment of a pedicle screwdisclosed herein.

FIG. 8 is a perspective view of an embodiment of a pedicle screwdisclosed herein.

FIG. 9 illustrates a pedicle screw disclosed herein implanted in apatient.

DETAILED DESCRIPTION

As illustrated in FIG. 1, embodiments disclosed herein are directed to apedicle screw which can include a screw body 10, a housing 12, a rod 14,a plurality of clamps 16, and a set screw 18. In some embodiments, oneor more of these components may be constructed from a metal, such astitanium or alloys thereof. For example, one or more components mayinclude titanium 6AL 4V ELI.

As shown in FIG. 1, in some embodiments, the rod 14 may be approximatelystraight. In other embodiments, the rod 14 may be curved. The rod 14 maybe of various lengths and diameters, for example, to span two or moreadjacent vertebrae. Those of ordinary skill in the art may appreciatethat any rod commonly used in spinal implantation may be used with thepedicle screw disclosed herein.

The pedicle screw disclosed herein may include a set screw 18, as shownin FIG. 1. In some embodiments, the set screw 18 may have externalthreading 60 as illustrated in FIG. 1 so as to be configured to bereceived by an internally threaded housing 12 as described below, thougha set screw may also be provided having internal threading to correspondwith an externally threaded housing. The set screw 18 may have squarethreads, though other types of threads are also contemplated. In someembodiments, the set screw 18 has an inner recess 62 which may beapproximately hexagonally-shaped. The inner recess 62 may be shaped andadapted to receive a torque wrench.

FIG. 2 illustrates a side view of a screw body 10 as described herein.The screw body 10 may have a proximal end 22 and a distal end 20. Asshown in FIG. 2, the distal end 20 may be at least partially threaded.In some embodiments, the distal end 20 of the screw body 10 may beadapted for implantation into the spine of a patient. For example, thedistal end 20 of the screw body 10 can be adapted for implantation intothe pedicle portion of a patient's vertebrae. As shown in FIG. 2, theproximal end 22 may include an enlarged head 24. In some embodiments,the enlarged head 24 may be approximately spherical, e.g., ball-shaped.The enlarged head 24 may have a roughened surface 26. As illustrated inFIG. 2, the roughened surface 26 may include threading, such as in theshape of a criss-cross pattern. In another embodiment, the threads maybe parallel to each other. In other embodiments, the roughened surface26 may include a sandblasted surface or the roughened surface 26 may begrooved. Those of ordinary skill in the art will appreciate that variousother roughened surfaces may be used. In other embodiments, the enlargedhead 24 may have an approximately smooth surface (not shown).

The pedicle screw disclosed herein may also include a housing 12, shownin FIGS. 3-4. The housing 12 may include an upper portion 38 having anupper opening 28 and a lower portion 40 having a lower opening 30. Insome embodiments, both the upper opening 28 and the lower opening 30 mayextend along a first axis 76 of the housing 12. The upper opening 28 andthe lower opening 30 may be connected so as to create a through holepassing from the upper portion 38 through the lower portion 40. Thediameter of the upper opening 28 may be larger than the diameter of theenlarged head 24 of the screw body 10 so that the screw can pass intothe housing through the upper opening 28. In some embodiments, thediameter of the lower opening 30 may be larger than the diameter of theenlarged head 24 of the screw body 10. In other words, the diameter ofthe enlarged head 24 may be smaller than that of the lower opening 30 ofthe housing 12. Thus, the screw body 10 may be loaded into the housing12 from either the upper opening 28 or the lower opening 30. In anembodiment, the screw body 10 may be loaded into the housing 12 from theupper opening 28. As shown in FIG. 4, the housing 12 may also include athird opening 32 and a fourth opening 34, both extending along a secondaxis 78 of the housing 12 which is transverse to the first axis, andintersecting with an upper edge of the housing 12. In some embodiments,the third opening 32 and the fourth opening 34 can be adapted to receivethe rod 14, with the width of the openings being about the same orslightly larger than the diameter of the rod. In an embodiment, both thethird opening 32 and the fourth opening 34 can be approximately“U”-shaped, though other shapes, such as an “O”-shape, may also be used.

As shown in FIG. 3, the upper portion 38 may define an interior recess42 surrounded by two approximately partially cylindrically-shaped walls66, with the openings 32 and 34 extending between the walls. The outersurface of the walls 66 may be adapted to couple to one or moreinsertion tools. As shown in FIG. 4, each wall 66 may have one or morecircular indentations 84 adapted to couple to a side hook holder. Eachwall 66 may also have a rectangular indentation 82 adapted to couple toan insertion tool, such as an alignment tube, rod fork, and persuader,and/or a tightening tool such as an anti-torque wrench.

At least part of the walls surrounding the interior recess 42 of theupper portion 38 may be internally threaded. In the embodimentillustrated, the threading 68 starts at the top of the housing 12, andterminates above the bottom of the third and fourth openings 32, 34. Insome embodiments, the threading 68 may not extend beyond a point at orbelow the rod 14 when it is resting in the housing 12 upon one or moreclamps 16, described below. An undercut region 80 may be provided belowthe threading 68. The internal threading may be adapted to receive andengage the set screw 18.

The housing 12 may have an intermediate portion 72 starting below theundercut region 80. As illustrated in FIG. 3, the inner wall of theintermediate portion 72 may be substantially cylindrical in shape, withthe openings 32 and 34 extending into the intermediate portion. Belowthe intermediate portion 72 there may be a lower portion 40. The lowerportion 40 may have a gradually decreasing diameter towards the bottomof the housing 12 along an interior surface 44. As shown in FIG. 3, theinterior surface 44 of the lower portion 40 may be curved. In anembodiment, the curved interior surface 44 may be non-spherical. Inother words, the curved interior surface 44 may have a center of radiusthat differs from its center of rotation. In other embodiments, theinterior surface 44 of the lower portion 40 may be approximatelyconical.

The pedicle screw described herein may have a plurality of clamps 16. Insome embodiments, the pedicle screw described herein may have two clamps16 placed side-by-side, for example, as shown in FIGS. 5 and 6. Althoughthe pedicle screw disclosed herein may be primarily described as havingtwo clamps 16, those of ordinary skill will appreciate that additionalclamps 16 may also be used. For example, in another embodiment, thepedicle screw may have four side-by-side clamps 16.

As shown in FIG. 5, each clamp 16 may have an upper portion 46 and alower portion 48. In the illustrated embodiment, the upper portion 46may have an indentation 50 adapted for receiving the rod 14, theindentation 50 being shaped approximately as a portion of a cylinder. Inan embodiment with two clamps 16, for example as shown in FIG. 5, eachindentation 50 may cooperate with each other to receive the rod 14. Theindentation 50 may be on a top surface of the upper portion 46. In otherembodiments, the indentation 50 may be on a side surface of the upperportion 46. As illustrated, the clamps 16 can be symmetrical. In otherembodiments, the clamps 16 can be asymmetrical. FIG. 5 shows anembodiment where the clamps 16 can be placed next to each otherhorizontally. There may be a plane 54 defined by the interface betweeneach clamp 16 that completely separates each ball clamp. In anembodiment with two clamps 16, each indentation 50 may be adapted toreceive the rod 14 such that the rod 14 is oriented parallel to theplane 54, as shown in FIG. 5. In another embodiment with two clamps 16,each indentation 50 may be adapted to receive the rod 14 such that therod 14 is perpendicular to the plane 54 (not shown). One or moreembodiments described herein may cause increased frictional engagementbetween the rod 14 and the clamps 16, resulting in a pedicle screw withoverall increased locking capability and integrity.

As shown in FIG. 5, the lower portion 48 of each clamp 16 may have aninner surface 52. In some embodiments, the inner surface 52 may beshaped as a concave approximately spherical, or ball-shaped, segment. Asshown in FIG. 5, each inner surface 52 may cooperate with each other toform a cavity 70, which may be approximately spherical or ball-shaped.The horizontal cross section of the cavity 70 may increase from the topof the cavity towards the middle, and may decrease from the middle tothe bottom of the cavity. The decreased cross section at the bottom ofthe cavity 70 may contribute to the engagement between the clamps 16 andthe enlarged head 24 of the screw body 10. The radius of the innersurface 52 may be larger than the radius of the enlarged head 24 of thescrew body 10. In other embodiments, the radius of the inner surface 52may be approximately the same as or smaller than the radius of theenlarged head 24 of the screw body 10. The inner surface 52 can beadapted to engage the enlarged head 24 of the screw body 10. In someembodiments, the inner surface 52 may be roughened. In an embodiment,the inner surface 52 may be threaded. In another embodiment, the innersurface 52 may be sandblasted (not shown). In yet another embodiment,the inner surface 52 may include a plurality of grooves, as illustratedin FIG. 5. Those of ordinary skill in the art will appreciate thatvarious other roughened surfaces may be used. In other embodiments, theinner surface 52 may be substantially smooth.

When assembled, a passageway 64 may extend from the collective upperportions 46 of each clamp 16 to the collective lower portions 48, asshown in FIG. 5. In some embodiments, the passageway 64 may provideaccess to the enlarged head 24 of the screw body 10 for implantationinto a patient's vertebrae. In these embodiments, the passageway 64 maybe used for driving the screw into the vertebrae when the screw hasalready been placed between the clamps 16 and within the housing 12.

Each ball clamp 16 has upper and lower outer surfaces 56, 58. The upperand lower outer surfaces 56, 58 of each clamp 16 may have a horizontalcross section that is approximately partly circular. For example, in anembodiment with two clamps 16, the upper and lower outer surfaces 56, 58of each clamp 16 may have a horizontal cross section that isapproximately a half-circle, as shown in FIG. 6. The outer surface 56 ofthe upper portion 46 may be approximately partially cylindrical inshape. In some embodiments, an exterior radius of the upper portion 46is smaller than an interior radius of the upper portion 38 of thehousing 12. The exterior radius of the upper portion 46 may also besmaller than the inner wall of the intermediate portion 72 of thehousing 12. In some embodiments, the upper portion 46 may also includeone or more indentations 74. The indentation 74 may be adapted toreceive a crimp 36, described further below. In some embodiments, eachclamp 16 may have one indentation 74. When arranged in the housing 12,the indentations may be aligned opposite each other, as depicted in FIG.5.

In some embodiments, as illustrated in FIG. 5, the outer surface 58 ofthe lower portion 48 gradually decreases in dimension towards the bottomof the clamp 16. As shown in FIG. 5, the outer surface 58 of the lowerportion 48 in one embodiment may be curved. In an embodiment, the outersurface 58 of the lower portion 48 may be non-spherical. In other words,the outer surface 58 of the lower portion 48 may have a center of radiusthat differs from its center of rotation. In other embodiments, theouter surface 58 of the lower portion 48 may be approximately partiallyconical in shape.

A cross-sectional view of an embodiment of a pedicle screw as partiallyassembled is shown in FIG. 7. The enlarged head 24 of the screw body 10may be adapted to fit at least partially within the inner surface 52 ofeach clamp 16. The clamps 16 may fit within the interior surface 44 ofthe lower portion 40 of the housing 12. In some embodiments, the radiusof curvature of the outer surface 58 of the lower portion 48 of theclamp 16 may be different from the radius of curvature of the interiorsurface 44 of the lower portion 40 of the housing 12. Advantageously,this difference in radii may allow for variation in the loading pointbetween the clamps 16 and the housing 12. For example, where the outersurface 58 of the lower portion 48 of the clamps 16 has a radius ofcurvature that is smaller than that of the interior surface 44 of thelower portion 40 of the housing 12, the result may be a loading pointthat is localized relatively close to the enlarged head 24 of the screwbody 10. In the alternate, the outer surface 58 of the lower portion 48of the clamp 16 may have a radius of curvature that is equal to that ofthe interior surface 44 of the lower portion 40 of the housing 12.Advantageously, these embodiments may result in a greater surface areaof contact between the clamp 16 and the housing 12, thus resulting inincreased stability. In some embodiments, the clamps 16 may contact eachother upon placement into the interior surface 44 of the lower portion40 of the housing 12. In other embodiments, the clamps 16 may notcontact each other. In these embodiments, the clamps 16 may be separatedby plane 54, as shown in FIG. 5.

The screw body 10, clamps 16, and housing 12 may be preassembled. Thescrew body 10 and the clamps 16 may be inserted into either the upperopening 28 or the lower opening 30 of the housing 12. In an embodiment,the clamps 16 may be loaded into the upper opening 28 of the housing 12and the screw body 10 may be loaded into the lower opening 30 of thehousing 12. In another embodiment, the clamps 16 can be placed on theenlarged head 24 of the screw body 10, and together they can be loadedinto the upper opening 28 of the housing 12. When assembled, one or moreclamps 16 may extend to a depth below the housing 12. In the embodimentillustrated in FIG. 7, when the pedicle screw is assembled, the enlargedhead 24 does not contact the inner surface 44 of the lower portion 40 ofthe housing 12, and the enlarged head has a smaller diameter than thesmallest inner diameter of the inner surface 44. While in this position,the screw body 10 may be capable of polyaxial movement relative to theclamps 16 and/or housing 12.

The indentation 50 on each clamp 16 may be aligned with the third andfourth openings 32, 34 of the housing 12 to accommodate rod 14. In thismanner, the rod may be positioned substantially parallel to the plane54. After inserting each ball clamp into position inside the housing,each clamp 16 may be held in place within the housing 12 and/or alignedvia an engagement member, such as crimp 36 made from the outside of thehousing 12 in the intermediate portion 72. The crimp 36 may be formed byapplying force to a selected portion of the housing 12 via a press andpunch apparatus, and may engage the indentations 74 on the sides of theclamps 16. In some embodiments, each clamp 16 may be capable of at leasta limited range of motion with crimp 36 in place.

With the clamps 16 and pedicle screw 10 held in position in the lowerportion of the housing, the rod 14 may be inserted into the third andfourth openings 32, 34 of the housing 12 and may be adapted to rest uponone or more clamps 16, as shown in FIG. 8. In some embodiments, thethreading on the interior recess 42 may not extend beyond a point at orbelow the rod 14 when it is resting upon one or more clamps 16. Uponinsertion, the rod 14 may be oriented approximately perpendicular to thescrew body 10. In the illustrated embodiment, the rod 14 does notdirectly contact the enlarged head 24 of the screw body 10.

The external threading 60 of the set screw 18 may be adapted to matewith the internal threading of the interior recess 42 of the upperportion 38 of the housing 12. As the set screw 18 is threaded into thehousing 12, the set screw 18 may contact and apply force to rod 14. Therod 14 may consequently engage the clamps 16, applying downward pressurethereto. The outer surface 58 of the lower portion 48 of each clamp 16may then engage the interior surface 44 of the lower portion 40 of thehousing 12. In some embodiments, the radii of both the outer surface 58and the inner surface 44 decrease towards the lower portions of eachclamp 16 and housing 12, respectively. In these embodiments, as theclamps 16 are pushed down into the housing 12, the inner surface 44 ofthe housing 12 may apply force to one or more outer surfaces 56, 58 ofeach clamp 16. Consequently, the inner surface 52 of each clamp 16 mayfrictionally engage the enlarged head 24 of the screw body 10. In someembodiments, the frictional engagement between each clamp 16 and theenlarged head 24 may be augmented via a roughened surface on the clamp16 and/or the enlarged head 24. As each clamp 16 increases frictionalengagement with the enlarged head 24, polyaxial movement of the screwbody 10 may be reduced, thus holding the screw body in place at aparticular orientation.

As shown in FIG. 9, the pedicle screw disclosed herein may be implantedin the vertebrae of a patient. In some embodiments the pedicle screw maybe implanted in the patient's thoracolumbar spine. In some embodiments,the distal end 20 of the screw body 10 may be adapted for implantationinto a pedicle of the patient's vertebrae. Prior to implantation, apathway for receiving the distal end 20 of the screw body 10 may becreated in the pedicle using methods known to those skilled in the art.In some embodiments, a preassembled combination of the screw body 10,clamps 16, and housing 12 may be inserted into the pathway. As mentionedpreviously, the enlarged head 24 of the screw body 10 may be accessedvia the passageway 64 between the clamps 16. The insertion tools may beadapted to correspond to the dimensions of the enlarged head 24 of thescrew body 10. In some embodiments, a polyaxial screwdriver may be usedto insert the distal end 20 of the screw body 10 into the pathway. Afterinsertion, the rod 14 may be placed into the third and fourth openings32, 34 using a rod holder. The set screw 18 may then be threaded intothe housing 12 and secured to lock the housing 12 and the rod 14 inplace at a chosen orientation. Various methods may be used to insert andsecure the set screw 18, as may be appreciated by those of ordinaryskill in the art. For example, various insertion tools may be used,including but not limited to an alignment tube, a rod fork, and apersuader. In some embodiments, the set screw 18 may be tightened usingan anti-torque wrench and a torque wrench. Advantageously, one or moreof the features described herein may contribute to an increased lockingstrength. Those of ordinary skill in the art may appreciate that aplurality of screw bodies 10 may be coupled to a single rod 14, and thatmultiple screw bodies 10 and rods 14 may be implanted in a patient.

The various pedicle screws and methods described above provide a numberof ways to carry out some preferred embodiments of the invention. Ofcourse, it is to be understood that not necessarily all objectives oradvantages described may be achieved in accordance with any particularembodiment described herein. Thus, for example, those skilled in the artwill recognize that the compositions may be made and the methods may beperformed in a manner that achieves or optimizes one advantage or groupof advantages as taught herein without necessarily achieving otherobjectives or advantages as may be taught or suggested herein.

Furthermore, the skilled artisan will recognize the interchangeabilityof various features from different embodiments. Similarly, the variouscomponents, features and steps discussed above, as well as other knownequivalents for each such component, feature or step, can be mixed andmatched by one of ordinary skill in this art to make compounds andperform methods in accordance with principles described herein.

Although the invention has been disclosed in the context of someembodiments and examples, it will be understood by those skilled in theart that the invention extends beyond these specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. Accordingly, the invention is notintended to be limited by the specific disclosures of preferredembodiments herein.

What is claimed is:
 1. A bone screw comprising: a threaded screw bodyhaving a proximal end and a distal end, the proximal end comprising anenlarged head; a housing having an upper portion having an upper openingand a lower portion having a lower opening extending along a first axisof the housing, and a third opening and a fourth opening along a secondaxis transverse to the first axis adapted to receive an elongated rod;and two clamps completely separated from each other by a plane andpositioned in the housing and having an upper surface adapted to engagethe elongated rod such that the rod is oriented parallel to the planeand a lower surface adapted to engage the enlarged head, wherein theplane is parallel to the second axis of the housing.
 2. The bone screwof claim 1, each clamp further comprising an indentation configured toreceive a crimp extending from an outside of the housing into engagementwith the indentation to hold the clamp in the housing, wherein an axisextending from one indentation to the other is perpendicular to theplane separating the two clamps.
 3. The bone screw of claim 1, whereinthe upper surface of each clamp comprises an indentation shaped toreceive the rod.
 4. The bone screw of claim 3, wherein the indentationis shaped as a portion of a cylinder.
 5. The bone screw of claim 1,wherein the clamps comprise an upper portion having a cylindrical outersurface.
 6. The bone screw of claim 1, wherein an outer surface of eachclamp has an approximately half-circular horizontal cross-section. 7.The bone screw of claim 1, wherein the lower portion of the housing hasa curved internal surface and the clamps comprise a lower portion havinga curved outer surface configured to engage the curved internal surfaceof the lower portion of the housing.
 8. The bone screw of claim 7,wherein the outer surface of the lower portion of the clamps isnon-spherical.
 9. The bone screw of claim 1, wherein each of the clampscomprises a lower portion having a curved inner surface adapted toengage the enlarged head.
 10. The bone screw of claim 1, wherein theclamps are symmetrical.
 11. The bone screw of claim 1, wherein thehousing comprises one or more indentations configured to couple to oneor more insertion tools.
 12. A kit comprising: the screw of claim 1; andat least one rod configured to engage the upper surfaces of the twoclamps.
 13. The kit of claim 12, wherein the at least one rod isstraight.
 14. The kit of claim 12, wherein the at least one rod iscurved.
 15. A method of assembling a bone screw comprising: positioningfirst and second clamps about an enlarged head of a threaded screw bodysuch that the first and second clamps are separated from each other by aplane; and inserting the first and second clamps and screw body into ahousing comprising an upper opening and a lower opening extending alonga first axis, and a third opening and a fourth opening along a secondaxis transverse to the first axis, wherein inserting the first andsecond clamps and screw body into the housing comprises inserting thefirst and second clamps and screw body together into the upper openingof the housing such that the plane separating the first and secondclamps is parallel to the second axis of the housing.
 16. The method ofclaim 15, further comprising forming a first crimp from an outside ofone side of the housing such that the first crimp extends intoengagement with an indentation in a first of the two clamps, and forminga second crimp from an outside of an opposite side of the housing suchthat the second crimp extends into engagement with an indentation in asecond of the two clamps such that that first and second crimps hold theclamps in place and an axis extending from one indentation to the otheris perpendicular to the plane separating the two clamps.